It is well known to use a multi-position switch in conjunction with heaters, for example radiant heaters, of electric cooking appliances. The heaters are provided with two or more heating elements which are connected in various configurations to give a plurality, for example six, of different heating power outputs. The heating elements may be connected in a number of different series and parallel arrangements, possibly with the use of rectifier means and/or a bimetallic switch device to adjust the power output of one or more of the heating elements and/or possibly with the application of more than one voltage to one or more of the heating elements to give different power outputs of the heating elements.
When heating elements, for example three heating elements, are used in the form of coils of bare resistance wire the effects of any visible changes in the radiation from the coils are small. This is because the coils increase and decrease slowly in brightness relative to a typical speed of rotation of a manually operated control knob of the multi-position switch and because the coils do not give off significant amounts of visible radiation at low power outputs.
More recently, radiant heaters have incorporated heating elements in the form of infra-red lamps. Initially four lamps were used, but subsequently radiant heaters with three, two and one infra-red lamp have been introduced. Although the major part of the radiation emitted by the infra-red lamps is in the infra-red region of the spectrum, a significant part of the radiation is visible. In the case of radiant heaters with four infra-red lamps it was relatively straightforward, by connecting the lamps in various parallel and series configurations, to obtain six progressive power output levels which corresponded with progressive visible radiation from the lamps. Moreover, the change in visible radiation and in power output is fast and virtually instantaneous at high power output levels.
For radiant heaters with three or two infra-red lamps it is also possible to provide a visible radiation level that corresponds substantially to the power output level of the heater, although in some cases it may be necessary to use additional means, such as a rectifier, to adjust the power output of the heater.
Thus the user of a cooking appliance that incorporates a radiant heater with heating elements in the form of infrared lamps has become accustomed to the heater providing a substantially accurate visual feedback of the power output of the heater.
However, in the case of a radiant heater with only a single infra-red lamp heating element the possibilities for series and parallel configurations is severely limited, even where the infra-red lamp heating element is used in conjunction with one or more heating elements in the form of coils of bare resistance wire. Nevertheless, there is a demand for a radiant heater with a single infra-red lamp heating element and this is coupled with an expectation that the visible radiation from the heater will change progressively with changes in power output of the heater.